Constitutive skin pigmentation dramatically affects the incidence of skin cancers, and in the US, rates of basal and squamous cell carcinomas and melanomas are dramatically higher in Whites than in Blacks. Cyclobutane pyrimidine dimers (CPD) and (6-4)-photoproducts (64PP), the two major types of DNA lesions resulting from UV exposure, are potentially carcinogenic. Melanin reduces DNA photoproducts in vivo and in vitro. Melanin content of melanocytes in culture plays a significant role in minimizing UV damage and in enhancing cell survival. However, much remains unknown about relationships between DNA damage/repair and different types, quantities, forms and distribution of melanin in skin of different racial/ethnic origin and different phototypes. Our collaborations with the FDA have been highly productive and have provided some insights into these basic questions. Several key observations have evolved from these studies: (1) DNA damage is greatest immediately following UV exposure in all subjects and was gradually repaired thereafter; (2) rates and efficiencies of removal of DNA lesions differs dramatically between subjects in all groups; (3) while DNA damage is most severe in light, UV-sensitive skin, even the darkest, UV-resistant skin incurs significant DNA damage after UV exposure at levels &#8804;1 MED; and (4) there is an inverse correlation between skin pigmentation and DNA damage. Thus, even very low UV exposures cause significant damage to DNA in all types of skin, underscoring the contention that there is no such thing as totally UV-resistant human skin. Our studies, combined with others on this topic, suggest that at least 5 factors contribute to UV-induced carcinogenesis and the various risks seen in human populations: (1) the amount of UV-induced DNA damage, (2) the identity and function of gene(s) damaged, (3) the nature of cells damaged, (4) the efficiency of post-exposure removal/repair of DNA damage, and (5) the removal of irreversibly UV-damaged cells by apoptosis or other mechanisms. The sum of those processes may explain the dramatically higher incidence of skin cancers in light-skinned subjects compared to dark-skinned subjects. We are in the process of completing our collaborative studies on effects of acute or repetitive UV exposure on human skin of various phototypes. Such work has gained urgency since repeated UV exposure by sunlamps (regulated by the FDA) has been linked with squamous cell carcinoma and cutaneous melanoma. Our results on effects of repetitive UV exposure further emphasize that distribution of melanin is important to the appearance of visible color and to photoprotection of the skin; however DNA damage is not eliminated during UV-induced pigmentation. Important issues to be resolved include defining: (1) whether production of eumelanin versus pheomelanin has any consequence on photoprotection, (2) whether strong facultative pigmentation provides added protection against UV damage, (3) the role of DNA repair in minimizing long-term damage to the skin and subsequent photocarcinogenesis, and (4) the identity and regulation of UV-induced factors that modulate responses to environmental stress. Given the importance of skin pigmentation to reduce the risk of photocarcinogenesis, these studies are critical to understand parameters determining photocarcinogenesis, as well as to develop effective strategies to minimize such risk. Our repetitive UV studies are now complete and are in the process of being published. We measured parameters that had been examined in the single UV dose study to determine levels of DNA damage and melanogenic functions in the skin during tanning elicited by repetitive UV exposure. Of the melanogenic proteins examined, increased expression of MITF occurs most quickly after UV exposure (within 1-2 d). Increased expression of melanosomal proteins such as TYR, TYRP1, Pmel17 and DCT is slower (1 wk), while increases in melanin synthesis take longer (3 wk) and increases in melanocyte density take even longer (4-5 wk). The distribution of melanin in skin is important to visible pigmentation and no doubt to photoprotective capacity. Although there is an initial surge (1 wk) in the upwards migration of existing pigment towards the surface of the skin, the balance in pigment distribution is restored within a few weeks when new synthesis of melanin has been established. It is clear that relatively small changes in melanin content and/or distribution can make relatively large changes in visible pigmentation. Those affect not only constitutive pigmentation that defines racial/ethnic differences, but also responses to UV exposure. The data from our FDA studies suggest that induction of DNA damage as a by-product of tanning is a significant source of delayed risks, including skin cancer. The induction/removal of DNA photoproducts in fair skin (phototypes 2 or 3) were measured for repeated UV exposures 1 day after the final exposure. Following repeated UV, redistribution of melanin to upper layers of the skin is an immediate response followed by de novo melanin synthesis. Based on diffuse reflectance measurements and melanin content assessed by FM staining, repeated UV doses produce a moderate to dark brown tan which reaches a plateau after several exposures, indicating that the pigment system reaches saturation. Pigmentation remains elevated for >4 wk after the final exposure showing that once a tan develops, continued frequent exposure is not necessary to maintain it. The 2-3 fold increase in melanin content of each layer of the epidermis elicited by repetitive UV exposure is consistent with the increased density of melanocytes in the basal layer of the skin. Surprisingly, higher cumulative repetitive UV doses resulted in lower levels of CPDs. We hypothesize that higher UV doses may have stimulated the cellular machinery to more efficiently repair DNA damage, and that the increased melanin protects against subsequent UV damage, but this needs to be further evaluated. We have also examined UV-irradiated specimens obtained 3 days after a 2 week course of repeated doses of SSR, UVA or UVB. It has been proposed that melanin protects against UVB more efficiently than against UVA, and that eumelanin absorbs UV more efficiently than pheomelanin, but those assumptions need to be tested. According to our UV reflectance analysis, different types of UV affect melanin content in human skin in different manners. We analyzed the relationship between CPD damage and melanin content using immunohistochemistry and Fontana Masson staining. Those results are consistent with our FDA studies and confirm that different subjects with apparently identical skin phototype have distinct responses to repetitive UV radiation. Some responded with significant increases in pigmentation (high SSR-response) and showed photoprotection against an SSR challenge, but low SSR-response subjects did not. Expression of melanocyt [summary truncated at 7800 characters]